1.) Field of the Invention
This invention relates to tomography and, more particularly, to near-field tomography wherein an image of an object is directly reconstructed with sub-wavelength resolution using only amplitude measurements.
2.) Description of the Background Art
There has been considerable recent interest in the development of methods which extend the spatial resolution of optical microscopy beyond the classical diffraction limit. Researches in near-field optics have provided a powerful set of approaches to directly address this problem. These approaches, which include near-field scanning optical microscopy (NSOM) and total internal reflection microscopy (TIRM), have been used to obtain subwavelength-resolved maps of the optical intensity near surfaces of effectively two-dimensional systems. However, when the sample presents a manifestly three-dimensional structure, interpretation of the resultant images has proven to be problematic. Recently, significant progress towards the development of three-dimensional near-field imaging has been made on two fronts. Nanotomography, a destructive method in which a sample is successively eroded and then imaged layer by layer with a scanning probe microscope, was reported in the article entitled xe2x80x9cNanotomographyxe2x80x9d by R. Magerle, Physical Review Letters, Vol. 85, No. 13, pgs. 2749-2752, Sep. 25, 2000. A nondestructive approach has also been devised and is based upon the solution to the linearized near-field inverse scattering problem for three-dimensional inhomogeneous media; this approach, entitled xe2x80x9cInverse Scattering for Near-field Microscopyxe2x80x9d, was reported by P. S. Carney and J. C. Schotland, Applied Physics Letters, Vol. 77, No. 18, pgs. 2798-2800, Oct. 30, 2000. For this latter method, the input data for the image reconstruction algorithm depends on the amplitude and phase of the scattered field. Measurements of the optical phase, particularly in the near field, are notoriously difficult since detectors generally record only intensities, necessitating the use of a holographic measurement scheme.
Thus, the art is devoid of a three-dimensional near-field microscopy technique which achieves subwavelength resolution without retrieval of the optical phase.
These shortcomings, as well as other limitations and deficiencies, are obviated in accordance with the present invention, referred to as as xe2x80x9cdual-beam internal reflection tomographyxe2x80x9d, by illuminating an object with an incident field composed of a coherent superposition of incoming evanescent waves, and by providing a direct reconstruction technique to an inverse scattering problem using measurements of output waves detected with and without the presence of the object. The superoscillatory properties of such waves may be used to encode the structure of the object on subwavelength scales.
In accordance with a broad method aspect of the present invention, a method for generating a tomographic image of an object includes: (a) probing the object with incident waves composed of a superposition of evanescent waves; (b) detecting the power extinguished from the incident waves by the object; and (c) reconstructing the tomographic image by executing a prescribed mathematical algorithm with reference to the incident waves and the extinguished power to generate the tomographic image with subwavelength resolution.
A broad system aspect of the present invention is commensurate with the broad method aspect.
The features of this approach are three-fold: (i) the near-field phase problem is circumvented by employing measurements of the power extinguished from the probe fields; (ii) the fields on which the power measurements are performed may be monitored far from the object and thus subwavelength resolution is obtained from far zone measurements; and (iii) by developing an analytic approach to the inverse problem in the form of an explicit inversion formula, an image reconstruction algorithm is produced which is strikingly robust in the presence of noise.